Metabolism of Phosphoacylglycerols
1. What Are Phosphoacylglycerols?
- Also called phospholipids.
- They contain:
- Glycerol backbone
- Two fatty acids
- A phosphate group linked to an alcohol (e.g., choline, ethanolamine, serine, inositol).
- They are major structural components of cell membranes.
2. Sites of Synthesis
- Synthesized mainly in the smooth endoplasmic reticulum (SER).
- Additional modifications may occur in the Golgi apparatus.
- Occurs in liver, brain, lung, and membranes of all cells.
3. General Pathway of Phospholipid Synthesis
- Starts with Glycerol-3-phosphate.
- Addition of two fatty acyl-CoA → forms Phosphatidic acid.
- Phosphatidic acid can form:
- Diacylglycerol (DAG) → base substituted to form lecithin, cephalin, etc.
- Or be converted into CDP-diacylglycerol → forms phosphatidylinositol, cardiolipin.
4. Metabolism of Lecithin (Phosphatidylcholine)
Synthesis
- Diacylglycerol (DAG) + CDP-Choline → Lecithin.
- Requires:
- Choline (diet or methylation of ethanolamine).
- S-adenosylmethionine (SAM) for methyl group transfers.
Functions
- Major phospholipid in cell membranes → maintains fluidity.
- Component of bile → helps solubilize cholesterol.
- Essential for lipoprotein assembly (VLDL formation).
- In lungs, lecithin (especially dipalmitoyl-phosphatidylcholine) is the primary pulmonary surfactant → prevents alveolar collapse.
Clinical Note
- Respiratory Distress Syndrome (RDS) in premature infants is due to insufficient lecithin production.
- Lecithin/Sphingomyelin ratio in amniotic fluid assesses fetal lung maturity.
5. Metabolism of Phosphatidylinositol 4,5-bisphosphate (PIP₂)
Synthesis
- Phosphatidylinositol (PI) undergoes sequential phosphorylation:
- PI → PIP → PIP₂.
Function: Key Signal Transduction Molecule
- When a hormone (e.g., acetylcholine, vasopressin, angiotensin II) binds to a Gq protein-coupled receptor:
- Phospholipase C (PLC) is activated.
- PLC cleaves PIP₂ → DAG + IP₃.
Roles of Hydrolysis Products
- IP₃ (Inositol triphosphate)
- Diffuses into cytoplasm.
- Binds ER receptors → releases Ca²⁺ → activates Ca²⁺-dependent enzymes.
- DAG (Diacylglycerol)
- Stays in membrane.
- Activates Protein Kinase C (PKC) → phosphorylates target proteins → regulates metabolism, secretion, cell growth.
Therefore, PIP₂ is a central molecule in hormone signaling pathways.
6. Regulation of Phospholipid Metabolism
- Controlled by the availability of fatty acids, choline, and inositol.
- Insulin stimulates phospholipid synthesis by promoting fatty acid synthesis.
- Cortisol and stress hormones increase phospholipid turnover during membrane repair and inflammation.
- Feedback regulation: Accumulation of phospholipids slows further synthesis.
7. Biological Importance of Phosphoacylglycerols
- Cell membrane structure → maintain fluidity and integrity.
- Surfactant in lungs → prevents alveolar collapse.
- Signal transduction (PIP₂ → IP₃ + DAG).
- Blood clotting (platelet phospholipids act as scaffolds for clotting reactions).
- Lipoprotein formation → important for lipid transport in blood.
- Myelin sheath formation → essential for nerve function.
- Serve as precursors for eicosanoids (prostaglandins, leukotrienes).